Modelling of continuous drying of heat-sensitive pharmaceutical granules in a horizontal fluidised bed dryer combined with a screw conveyor at steady state

Abstract

• A novel FBD has been investigated for continuous drying of pharmaceutical granules. • A relatively simple and practical model has been developed to predict the continuous dryer operation. • This model contributes to facilitating process scaling up and gaining insight on process optimization strategies. • Usefulness of the work relies on the need of process intensification through continuous manufacturing in pharmaceutical industry. Continuous manufacturing improves productivity in terms of efficiency, flexibity and quality. However, the pharmaceutical industry still heavily relies on batch operations; extensive R&D work is required to convert the current processes into continuous ones. This work presents a phenomenologigcal modelling approach for continuous drying of pharmaceutical granules in a novel fluidised bed dryer (FBD) combined with a screw conveyor. A hybrid model methodology has been adopted to describe the drying kinetics of the target granules: (1) a phenomenological model for the constant-drying rate period and (2) an empirical one for the falling rate period. The model was first validated by batch drying results and then was extended to the corresponding continuous drying condition. The particle residence time distributions (RTDs) have been determined by the pulse-tracer injection method. Then, a dispersion model has been developed based on the RTD measurements and the drying kinetics that allows predicting the continuous drying results at a steady state. The solids flow has been considered as a plug flow moving from the inlet to outlet with moderate axial dispersion, and the back-mixing term has been expressed through an experimental dispersion coefficient. The influences of several parameters are studied towards process optimisation and scale-up. The increase of air temperature and velocity were efficient on reducing the final moisture content. The increase of the feeding rate enhanced the energy efficiency. And the increase of the internal rotation rate enhanced the process stability and reduced the overheating effect.